what is the error message given when there is a credit dot?

Sounds like a trough opto issue. Like the last ball isn't rolling down into the trough properly to break the opto beam.

When you go to start a new game and it says pinball missing. Give the game a bit of a shake sideways to see if the last ball rolls further down the trough. The rails in the trough can develop divots over time and might need to be filed smooth.